Undesired wrinkles in skin are commonly seen in dermatologic practice. Wrinkles in skin may be caused by age and by exposure to the sun's ultraviolet rays. Human skin consists mainly of two layers: the top layer of skin known as the epidermis; and the layer beneath the epidermis known as the dermis. The dermis is primarily a cellular and is composed of water, the protein collagen, and glycosaminoglycans. Water constitutes approximately 70 percent of the total weight of the dermis. Collagen constitutes approximately 70 percent of the dry weight of the dermis, and glycosaminoglycans constitute between approximately 0.1 and 0.3 percent of the dry weight of the dermis. Collagen and glycosaminoglycans are constantly produced by fibroblasts, a type of connective tissue cell, and degraded by enzymes. Collagen degradation relies primarily on specific proteinases known as collagenases.
Collagen provides the dermis with the majority of its structural integrity. With aging, the amount of dermal collagen decreases and is replaced by the protein elastin. In addition, the remaining collagen tends to be chaotically oriented as compared to the more organized patterns found in youthful skin. Glycosaminoglycans are very hydrophilic, and increased amounts of these carbohydrates are associated with the increased skin vigor found in youthful skin. One major difference between the smooth, supple skin of newborns and the drier, thinned skin of older individuals is the far greater relative amount of glycosaminoglycans found in newborn skin. The glycosaminoglycans found in newborns can bind up to 1000 times their weight in water. As the skin ages and the amount of glycosaminoglycans decreases, the skin may become less hydrated and lose some of the suppleness found in youth. Also, the remaining glycosaminoglycans in photo-aged skin are deposited on the haphazardly arranged elastin fibers which have replaced the collagen fibers. The placement of the remaining glycosaminoglycans may partially account for the weather-beaten appearance of photo-aged skin.
Existing procedures for eliminating or reducing the severity of wrinkles include chemical peels, mechanical abrasion and laser ablation. All of these methods remove the top layer of skin. A new top layer forms during healing. Cosmetic improvement is seen when the skin containing wrinkles is replaced by a new layer of horizontally oriented neocollagen in the superficial dermis. However, all of these methods disrupt and completely remove the epidermis. The resulting open wounds require daily care to optimize wound healing. Epidermal destruction and subsequent healing has several undesirable side effects. These undesirable side effects include prolonged hypopigmentation, hyperpigmentation, erythema and edema. Hyperpigmentation occurs frequently in darker skin types as a result of an inflammatory response of the skin. Hyperpigmentation results in the treated area of the subject's skin turning darker than the surrounding untreated skin. Hyperpigmentation can be slow to clear, sometimes taking up to a year to disappear. Hypopigmentation is attributable to damage to the melanin-producing cells in the skin. While generally transient, hypopigmentation can be permanent, and is cosmetically undesirable while it persists. Also, erythema or redness of the skin may be significant for weeks to months after the procedure, requiring the patients to wear conspicuous amounts of make-up.
A known property of collagen fibers, such as those found in the skin. is that the fibers shrink when elevated to a temperature in the range of 60 to 70 degrees Celsius, which is about 30 degrees Celsius above normal body temperature. Temperature elevation ruptures the collagen ultrastructural stabilizing cross-links, and results in immediate contraction in the collagen fibers to about one-third of their original length without changing the structural integrity of the fibers. One known technique shrinks the collagen fibers in the cornea of the eye to change the shape of the cornea and correct refractive disorders. This technique involves the use of laser energy in a wavelength range of about 1.80 to about 2.55 microns. The laser energy is used to irradiate the collagen in the cornea to elevate the collagen to at least 23 degrees Celsius above normal body temperature and thereby achieve collagen shrinkage. U.S. Pat. Nos. 4,976,709, 5,137,530, 5,304,169, 5,374,265, and 5,484,432 to Sand disclose a technique and apparatus for controlled thermal shrinkage of collagen fibers in the cornea.
However, this technique cannot be effectively used to remove wrinkles in skin by shrinking dermal collagen. The bulk of the shrunken, thermally denatured, collagen fibers do not remain in the skin after treatment with this technique. Unlike the cornea, which is avascular, an aggressive healing response in the skin degrades the denatured collagen in the superficial dermis by collagenases, thereby rapidly eliminating the bulk of the shrunken collagen from the skin.
Additionally, in the 1.80 to 2.55 micron wavelength range, strong absorption of the laser energy by water present in the skin limits the penetration depth of the laser radiation to a small fraction of a millimeter. The depths of thermal injury which can be achieved in skin using the wavelengths in this range are therefore limited to the most superficial layer of the skin. Such superficial injury leads to an inflammatory healing response characterized by prolonged visible edema and erythema, as well as the possibility for long lasting pigmentary disturbances.